The present application relates to a method of manufacturing an optical sheet made of an amorphous state transparent crystalline resin sheet whose surface is subject to emboss processing.
A so-called emboss sheet has been manufactured which is a resin sheet or film whose surface is formed with a regular steric geometric design (emboss pattern). Typically, a melt extrusion method is widely used in which thermoplastic resin melt is pushed out of a T die in a sheet shape, the melt sandwiched and compressed between a metal roll having a concave/convex shape on the circumference surface thereof and a rubber roll is cooled and solidified so that a sheet having a concave/convex shapes on a surface and a flat rear surface is continuously formed (e.g., refer to Japanese Patent Unexamined Publication No. HEI-9-295346, Patent Document 1).
With the melt extrusion method, transfer and detach are performed at the same time for resin pushed out of the T die by using the same solid roll having a geometric shape. To make transfer perfect, it is necessary that resin has sufficient thermal energy, and to perform detachment, it is necessary that the resin is cooled down to not higher than a glass transition temperature (Tg) of the resin. Since the melt extrusion method makes transfer and cooling by using the same solid roll, sufficient heating and cooling is difficult to be performed and it is difficult to perform both transfer and detachment perfectly.
According to another emboss sheet manufacture method, an emboss pattern formed on the surface of a metal roll or a metal flat plate is transferred to the surface of a resin sheet. According to another known method, an emboss pattern is formed on the surface of a resin sheet by using a metal endless working belt wound around a plurality of rolls and having the emboss pattern formed on the surface of the endless belt (e.g., refer to Japanese Patent Unexamined Publication No. 2001-277354, Patent Document 2).
An emboss sheet manufactured in the manner described above can be used, for example, as an optical sheet for a liquid crystal display apparatus. Specifically, a prism sheet in which a prism shape of a triangle shape in section disposed consecutively can be used as the emboss sheet. The prism sheet is widely known as a luminance improving sheet (film) for improving a front luminance by converging backlight. For example, WO2006/071621 Publication (Patent Document 3) discloses a prism sheet having in-plane anisotropy of a refractive index and formed by stretching a resin sheet having a prism shape on the surface thereof.
It may be required to form an amorphous state resin sheet having an emboss pattern on the surface thereof. In order to make an emboss sheet subject to shape processing have in-plane anisotropy of a reflective index, a crystalline resin sheet is typically stretched along a uniaxial direction or biaxial directions. In this case, it is preferable that a crystalline resin sheet is in an amorphous state so that a stretching process can be executed properly at a higher precision.
However, with the related art emboss sheet manufacture methods described above, it is very difficult to execute an embossing processing while the resin sheet is maintained in the amorphous state. Namely, with the related art emboss sheet manufacture methods, it is not possible to prevent resin from being crystallized during a cooling process in which the temperature of the resin sheet is lowered down to a detachment temperature after the resin sheet is formed with the emboss pattern by raising the temperature higher than the glass transition temperature or in the vicinity of crystallization temperature range. As crystallization of the resin sheet precedes, resin is whitened and its transparency is lost, and consequently the resin sheet becomes not suitable for use as the optical sheet. If an emboss pattern transfer temperature is low or a detachment temperature is high, it is not possible to obtain a high emboss pattern transfer precision.